1. Field of the Disclosure
The disclosure relates generally to the design and use of catalytic membrane reactors. More specifically, the disclosure relates to reactors containing one or more oxygen transport membranes and a catalyst, and methods of using the same for carrying out more efficient chemical reactions.
2. Brief Description of Related Technology
Catalytic membrane reactors using solid state membranes for oxidation and/or decomposition of various chemical compositions have been studied and used. One potentially valuable use of such reactors is in the production of synthesis gas. Synthesis gas is a mixture of carbon monoxide (CO) and molecular hydrogen (H2), and is used as a feedstock in the production of bulk chemicals such as, for example, methanol, acetic acid, ammonia, oxo-products, hydrogen, hydroquinone, ethanol, ethylene, paraffins, aromatics, olefins, ethylene glycol, Fischer-Tropsch products, substitute natural gas, and other liquid fuels, such as gasoline.
Synthesis gas typically is produced from natural gas (i.e., gas containing methane (CH4)) or other light hydrocarbons by steam reforming or partial oxidation. In steam reforming, natural gas is mixed with steam and heated to high temperatures. Thereafter the heated mixture is passed over a catalyst, such as nickel (Ni) on aluminum oxide (Al2O3), to form synthesis gas:CH4+H2O→3H2+COSynthesis gas is obtained in a partial oxidation reaction when natural gas is reacted with molecular oxygen (O2) in an exothermic reaction (i.e., the reaction evolves energy):CH4+½O2→2H2+CO
Both the steam-reforming reaction and the partial oxidation reaction are expensive to maintain. Conventional steam-reforming techniques present significant obstacles. First, the chemical reaction to produce the synthesis gas from natural gas and steam (H2O) is endothermic (i.e., the reaction requires energy). Roughly one third of the natural gas consumed in the steam-reforming process is required to produce the heat necessary to drive the endothermic reaction, rather than to produce the synthesis gas. Second, the ratio of H2:CO in the synthesis gas produced by steam reforming is relatively high (e.g., about 3:1 to about 5:1). For most efficient use in the synthesis of methanol, for example, the ratio of H2:CO in synthesis gas should be about 2:1. Adjusting this ratio, however, adds to the cost and complexity of the process. In the partial oxidation reaction, significant energy and capital are required to provide the molecular oxygen necessary to drive the reaction. The oxygen typically is obtained through capital intensive air-separation units.
Catalytic membrane reactors are valuable in the production of synthesis gas. In a catalytic membrane reactor that facilitates oxidation/reduction reactions, a catalytic membrane separates an oxygen-containing gas from a reactant gas which is to be oxidized. Oxygen or other oxygen-containing species (e.g., NOx or SOx) are reduced at a reduction surface of the membrane to oxygen ions (O2−) that are then transported across the membrane to its other surface, in contact with the reactant gas. The reactant gas, for example methane, is oxidized (e.g., from CH4 to CO) by the oxygen ions, and electrons (e−) are evolved at the oxidation surface of the membrane. Use of these catalytic membrane reactors is believed to be beneficial for a number of reasons. First, the reaction to produce synthesis gas mediated by the catalytic membrane reactor (CH4+½O2→2H2+CO) is exothermic, as noted above. The evolved heat can be beneficially recovered in a co-generation facility, for example. Second, the synthesis gas produced using the catalytic membrane reactor can produce synthesis gas having a H2:CO ratio of about 2:1. Thus, the additional and expensive processing steps necessary in conventional steam-reforming techniques are obviated, and all of the consumed natural gas can be used to produce synthesis gas.
Membrane materials useful in separating oxygen from oxygen-containing gases generally are mixed conductors, which are capable of both oxygen ion conduction and electronic conduction. The driving force of the overall oxygen transport rate through the membrane is the different oxygen partial pressure applied across the membrane. Suitable membranes are dense and gas-impermeable. Thus, direct passage of oxygen molecules and any other molecular species through the membrane is blocked. Oxygen ions, however, can migrate selectively through the membrane. The membrane thus separates oxygen from other gases.
More specifically, at elevated temperatures, generally in excess of 400° C., suitable membrane materials contain mobile oxygen ion vacancies that provide conduction sites for the selective transport of oxygen ions through the membrane. The transport through the membrane material is driven by the ratio of partial pressure of oxygen (Poxygen) across the membrane, where oxygen ions flow from a side with high Poxygen to a side with low Poxygen. Dissociation and ionization of oxygen (O2 to O2−) occurs at the membrane cathode (or reduction) surface where electrons are picked up from near surface electronic states. The flux of oxygen ions is charge-compensated by a simultaneous flux of electronic charge carriers in the opposite direction. When the oxygen ions travel through the membrane and arrive at the opposite anode (or oxidation) surface of the membrane, the individual ions release their electrons and recombine to form oxygen molecules, which are released in the reactant gas stream and the electrons return to the other side through the membrane.
The permeation or diffusion rate (also referred to herein as “flux”) through a non-porous ceramic membrane is controlled by (a) the solid-state ionic transport rate within the membrane, and (b) the ion surface exchange rate on either side of the membrane. The flux of the gas to be separated usually can be increased by reducing the thickness of the membrane, until its thickness reaches a critical value. At above this critical value, the oxygen flux is controlled by both the ion surface exchange kinetics and solid state ionic transport rate. Below the critical thickness, the oxygen flux is mainly dominated by its ion surface exchange rate. Therefore, thinner membranes are desirable due to their higher solid state ionic transport rate than are thicker membranes. However, a lower ion surface exchange rate (i.e., a higher surface resistance to transport rate) of thinner membranes, can become dominant in the overall component transport rate. Surface resistance arises from various mechanisms involved in converting the molecules to be separated into ions or vice-versa at both surfaces of the membrane.
Oxygen ion conductivity in a material can result from the presence of oxygen ion defects. Defects are deviations from the ideal composition of a specific material or deviations of atoms from their ideal positions. One mechanism of oxygen ion conduction in a material is “jumping” of oxygen ions from site-to-site where oxygen vacancies exist. Oxygen vacancies in a material facilitate this “jumping” and, thus, facilitate oxygen ion conduction. Oxygen ion defects can be inherent in the structure of a given material of a given stoichiometry and crystal lattice structure, or created in a membrane material through reactions between the membrane material and the gas to which it is exposed under the operating conditions of the catalytic membrane reactor. In a given system with a given membrane material, both inherent and induced defects can occur.
Materials with inherent oxygen anion vacancies are generally preferred for use as the membrane. Loss of oxygen from a membrane material by reaction to create vacancies typically has a detrimental effect on the structural integrity of the material. As oxygen is lost, the size of the crystal lattice increases on a microscopic level. These microscopic changes can lead to macroscopic size changes. Because membrane materials are brittle, size increases lead to cracking making the membrane mechanically unstable and unusable. Furthermore, the cracking and size changes can undesirably render a once gas-impermeable material gas permeable.
Catalysts useful in the production of synthesis gas are known, and have been coated onto surfaces of membranes in the past such as, for example, in Mazanec et al. U.S. Pat. Nos. 5,714,091 and 5,723,035, and in Schwartz et al. U.S. Pat. No. 6,214,757. Generally, such catalysts include, but are not limited to, cobalt and nickel on aluminum oxide or magnesium oxide. These catalysts, however, have not necessarily been used in combination with catalytic membrane reactors for the production of synthesis gas.
The beneficial use of catalytic membrane reactors is not limited to the conversion of natural gas to synthesis gas. These reactors also can be used where oxides of nitrogen (NOx) and sulfur (SOx) and hydrogen sulfide (H2S) are decomposed, such as disclosed in the '757 patent.
There are a number of significant challenges in constructing and maintaining catalytic membrane reactors not adequately addressed in the prior art. For example, membrane materials must be capable of conducting oxygen ions while also being chemically-and mechanically-stable at the high operating temperatures and other harsh conditions experienced during reactor operation. Further, the membrane material must remain non-reactive or inert with respect to the various catalyst material within the reactor used to catalyze the chemical reaction. Still further, the membrane material must remain non-reactive or inert with respect to the various non-oxygen-containing reactants within the reactor consumed in the chemical reaction. Additionally, provisions should be made in the reactor for electronic conduction to maintain membrane charge neutrality.